This invention relates to a method for the manufacture of an investment shell mold into which an alloy composition is to be cast for producing a super alloy which consists mainly of nickel or cobalt and which has the grains of its crystal structure oriented in a single direction.
Heretofore, investment molds have been manufactured by the embedding method or the shell method. This invention pertains to the investment mold by the shell method. Generally, this mold is produced by applying to the surface of a pattern formed of a soluble substance like wax or an inflammable substance a layer of slurry obtained by adding colloidal silica or ethyl silicate hydrolyzate, for example, as a binder to a refractory powder such as alumina, zirconia, or fused silica and, after the applied layers have set, removing the pattern by melting or burning. The mold thus produced is mostly used for precision casting of metallic materials which are complicated in shape or difficult to cut.
The mold manufactured by the method of this invention is suitable for production of super alloys made up of grain-oriented crystals as described above. The reason for this suitability is as follows.
In the production of super alloys, various efforts have been made to impart to these alloys with improved mechanical strength as by controlling, while the molten super alloy composition is in the process of being cooled to be set, the conditions of cooling thereby causing the alloy to solidify with the crystal thereof oriented in one direction. These efforts have resulted in development of various grain-oriented solidified super alloys. A typical alloy composition is shown below. In Ni-based grain-oriented solidified super alloys, such highly active elements as Al, Tl, Mo, W, and Ta are included for the purpose of fortifying the matrix and enabling formation of high-temperature hardening phase.
______________________________________ Typical composition of grain-oriented solidified Ni--based super alloy Cr Mo W Al Ti Ta Ni ______________________________________ 9.0 1.0 10.5 5.8 1.2 3.3 Bal. ______________________________________
For the purpose of casting an alloy composition containing such highly active elements as described above to produce a casting made up of crystal grains oriented in a single direction, however, the investment mold obtained by the conventional shell method is unsuitable because it produces free silica.
The reason for this will be explained in more detail.
When the mold is manufactured, the silica contained in the coating material as a binder collects on the cavity surface of the completed mold. Then, when the mold having silica on its cavity is used for solidifying the molten alloy composition into a grain-oriented alloy, the silica reacts with the active elements of the alloy, thus changing the composition of the alloy. It also tends to prevent the formation of grains oriented in a single direction.
To preclude this problem, therefore, the cavity face of the mold is required to be formed of a refractory substance virtually incapable of reacting with the alloy components even when the face is exposed to contact with the molten alloy composition. In the production of the conventional investment shell mold, therefore, there has been applied to the cavity face a face coating of a slurry obtained by combining a refractory powder such as alumina and a binder such as colloidal silica or ethyl silicate hydrolyzate.
This method, however, has the disadvantage that free silica nevertheless finds its way to the cavity face.
An object of this invention is to provide a method for the manufacture of an investment shell mold in which free silica does not appear on the cavity face and, therefore, which is suitable for the production of a grain-oriented solidified super alloy.